The present invention relates to analogue to digital converters, methods of analogue to digital conversion, in particular analogue to digital conversion using clip processors adapted to regenerate a clipped signal, produced when an analogue signal exceeds the dynamic range of an analogue to digital converter, and to clip processors for use with analogue to digital converters.
The ability to perform digital signal processing has had, and continues to have, a major impact on todays society. Central, to digital signal processing, is the conversion of analogue signals to a digital format. This process is implemented with an analogue-to-digital converter (ADC), an electronic circuit that takes an analogue signal as input and gives a digital representation of the analogue signal as an output. The process of analogue to digital conversion employs the steps of:
sampling the analogue signal to split the continuous signal up into a set of discrete components along the time axis;
quantising the sampled signals by comparing them to a set of discrete signal values; and
digitising the quantised signal to produce a binary word representative of the analogue sample.
ADCs have a limited amplitude, or dynamic, range for which they can produce an accurate digital representation, however quantised, of the input signal. If the input signal exceeds the dynamic range of an ADC, the digital representation will no longer correspond to the analogue input but will only indicate that the analogue signal is outside the dynamic range. This phenomenon is called xe2x80x9cclippingxe2x80x9d. The digital version of an analogue signal, which exceeds the dynamic range of the ADC used to digitise it, is a poor representation of that signal. However, the output of the ADC will give an indication that the dynamic range of the ADC has been exceeded and that signal clipping has occurred.
The present invention uses this information to improve the digital representation of the analogue signal in the sense that the correspondence between the input and output signals is improved.
According to a first aspect of the present invention, there is provided a circuit, including an analogue to digital converter for digitising an analogue signal, characterised in that said circuit includes a clip processor, adapted to estimate a value for clipped digital signal samples and means for replacing said digital signal samples with said estimated values, thereby mitigating the effects of clipping in an output from said circuit.
Said means for replacing may be a buffer adapted to dynamically store a plurality of digitised samples produced by said analogue to digital converter, and said clip processor may be adapted to read digitised samples from said buffer and replace said clipped digitised samples with estimates of unclipped values thereof.
Said analogue to digital converter may be adapted to produce a signal indicative of analogue to digital converter saturation, said clip processor may be adapted to receive said signal, and said clip processor may be adapted, on receipt of said signal, and for so long as said signal is present, to estimate values for clipped digital signal samples and to cause said means for replacing to replace said digitised samples with estimates of unclipped values thereof.
Said clip processor may be adapted to detect saturation of said analogue to digital converter by comparing a digitised sample produced by said analogue to digital converter to maximum and minimum values for said analogue to digital converter""s dynamic range.
Said means for replacing may be incorporated within said clip processor.
Said buffer may be a shift register.
Said buffer may be of a length sufficient to store:
a first plurality of unclipped samples occurring before a group of clipped samples;
said group of clipped samples; and
a second plurality of unclipped samples following said group of clipped samples.
Said first and second plurality of samples may include identical numbers of samples.
Said first and second pluralities of samples may each comprise three samples.
Said clip processor may be adapted to measure the number of samples in a consecutive group of clipped samples.
Said clip processor may be adapted to estimate a value for each sample in said consecutive group of clipped samples from the number of samples in said group, the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples and the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples.
A digital signal may have a higher bit resolution after processing by said clip processor than it had prior to processing by said bit processor.
Said clip processor may be adapted to estimate a value for each sample in said consecutive group of clipped samples from:
the number of samples in said group;
the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples;
the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples;
a normalised clip level, or crest factor; and
an autocorrelation factor of a channel over which said analogue signal has been transmitted.
Said clip processor may be adapted to estimate a value for each sample in said consecutive group of clipped samples from:
the number of samples in said group;
the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples;
the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples;
a normalised clip level, or crest factor; and
an autocorrelation factor of said analogue to digital converter.
Said first plurality of samples may include a single sample contiguous with said group of clipped samples and said second plurality of samples may include a single sample contiguous with said group of clipped samples.
Said clip processor may feed ncut (where ncut is the number of samples in a group of clipped samples), coefficient vectors, ak, sequentially, to a vector multiplier which performs one scalar product per clipped sample to be estimated, and said scalar multiplier may be adapted to multiply each sample, from a group of unclipped samples contiguous with a group of clipped samples, with a coefficient vector belonging to a sample to be estimated.
An output from said vector multiplier may be passed through a non-linearity to ensure that estimates of clipped samples stay above a clip level.
According to a second aspect of the present invention, there is provided, a digital receiver, characterised in that said digital receiver includes a circuit, including an analogue to digital converter, for digitising a received analogue signal as set forth in any preceding paragraph.
According to a third aspect of the present invention, there is provided a DMT transmission system, characterised in that said DMT transmission system includes a plurality of receivers as set forth in the preceding paragraph.
According to a fourth aspect of the present invention, there is provided a method of digitising an analogue signal, in which said analogue signal is subjected to the steps of:
sampling;
quantisation; and
digitisation;
characterised by the steps of:
buffering samples of said digitised signal;
detecting whether said samples are clipped;
determining a number of consecutive clipped samples, constituting a group of clipped samples;
reading a first plurality of unclipped samples preceding said group of clipped samples into a buffer, reading said group of clipped samples into said buffer and reading a second plurality of unclipped samples into said buffer;
computing an estimate of the unclipped values of said group of clipped samples; and
overwriting said group of unclipped samples with the estimated values thereof.
According to a fifth aspect of the present invention, there is provided a method of analogue to digital conversion, characterised by estimating values for clipped digital signal samples and replacing said clipped digital signal samples with said estimated values, thereby mitigating the effects of clipping.
A plurality of digitised samples, produced by an analogue to digital converter, may be dynamically stored in a buffer, digitised samples may be read from said buffer into a clip processor and said clipped digitised samples, in said buffer, may be replaced with estimates of unclipped values thereof.
Said analogue to digital converter may produce a signal indicative of analogue to digital converter saturation, said clip processor may receive said signal, and said clip processor, on receipt of said signal, and for so long as said signal is present, may estimate values for clipped digital signal samples and cause said digitised samples to be replaced with estimates of unclipped values thereof.
Said clip processor may detect saturation of said analogue to digital converter by comparing a digitised sample produced by said analogue to digital converter to maximum and minimum values for said analogue to digital converter""s dynamic range, and may estimate values for clipped digital signal samples and cause said digitised samples to be replaced with estimates of uncipped values thereof, on detection of saturation, and for so long as saturation is detected.
Said buffer may have a length sufficient to store:
a first plurality of unclipped samples occurring before a group of clipped samples;
said group of clipped samples; and
a second plurality of unclipped samples following said group of clipped samples.
Said first and second plurality of samples may include identical numbers of samples.
Said first and second pluralities of samples may each comprise three samples.
Said clip processor may measure the number of samples in a consecutive group of clipped samples.
Said clip processor may estimate a value for each sample in said consecutive group of clipped samples from the number of samples in said group, the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples and the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples.
A digital signal may have a higher bit resolution after processing by said clip processor than it had prior to processing by said bit processor.
Said clip processor may estimate a value for each sample in said consecutive group of clipped samples from:
the number of samples in said group;
the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples;
the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples;
a normalised clip level, or crest factor, and
an autocorrelation factor of a channel over which said analogue signal has as been transmitted.
Said clip processor may estimate a value for each sample in said consecutive group of clipped samples from:
the number of samples in said group;
the values of samples in a consecutive group of samples immediately preceding said consecutive group of clipped samples;
the values of samples in a consecutive group of samples immediately following said consecutive group of clipped samples;
a normalised clip level, or crest factor; and
an autocorrelation factor of said analogue to digital converter.
Said first plurality of samples may include a single sample contiguous with said group of clipped samples and said second plurality of samples may include a single sample contiguous with said group of clipped samples.
Said clip processor may feed ncut (where ncut is the number of samples in a group of clipped samples), and coefficient vectors, ak, sequentially, to a vector multiplier which performs one scalar product per clipped sample to be estimated, and by said scalar multiplier multiplying each sample, from a group of unclipped samples contiguous with a group of clipped samples, with a coefficient vector belonging to a sample to be estimated.
An output from said vector multiplier may be passed through a non-linearity to ensure that estimates of clipped samples stay above a clip level.